The present invention relates generally to an optical system, and more particularly to a detection optical system in an exposure apparatus that exposes various plates, such as a semiconductor wafer single crystal substrate, and a liquid crystal display (“LCD”) glass plate. The present invention is suitable, for example, for an optical system in an exposure apparatus that utilizes the ultraviolet (“UV”) light and the extreme ultraviolet (“EUV”) light as an exposure light source.
A conventional reduction projection exposure apparatus uses a projection optical system to project a circuit pattern of a mask (or a reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing the fine semiconductor device using the photolithography technology.
The transferable minimum critical dimension (or the resolution) of the projection exposure apparatus is proportionate to a wavelength of the exposure light, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the finer the resolution is. Along with the recent demands for finer processing to semiconductor devices, use of a shorter wavelength of the exposure light has been promoted in the exposure light source from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm). Thus, the exposure light comes to use the UV light with a shorter wavelength for the exposure light.
Nevertheless, the lithography using the UV light has the limit to satisfy the growing demands for finer processing to the semiconductor devices. An EUV exposure apparatus has been developed to effectively transfer a very fine circuit pattern of 42 nm or smaller. The EUV exposure apparatus uses the EUV light having a wavelength between 10 to 15 nm shorter than that of the UV light.
As the light becomes highly likely to be absorbed in a material as its wavelength becomes shorter. It is impractical to use a refractive element or lens, which is applicable to the visible and UV lights. There are no glass materials usable for the EUV light wave range, and a catoptric optical system that consists of reflective elements or mirrors should be used.
Since the EUV light is likely to be absorbed or attenuated in the air atmosphere, the EUV exposure apparatus should be used in the vacuum atmosphere. Accordingly, the EUV exposure apparatus houses at least an optical path that the EUV light travels in a vacuum chamber that maintains the vacuum atmosphere, preventing the attenuation of the EUV light while it is in use.
An alignment detection system and a focus position detection system do not use the EUV light, and thus can place some of the components in the air atmosphere via a glass or a viewing window that isolates the vacuum atmosphere from the air atmosphere. See, for example, Japanese Patent Application, Publication No. 2001-217191. This reference arranges, in the air atmosphere, those components which would malfunction in the vacuum atmosphere, or cause contaminants and thus cannot be arranged in the vacuum atmosphere, such as a measuring light source, an imaging sensor, and an electric circuit.
On the other hand, all or rest of such optical systems as an alignment detection system and the focus position detection system are arranged in the vacuum atmosphere. When the environment changes from the air atmosphere to the vacuum atmosphere, the refractive index of the atmosphere in which the optical system is placed varies, and a problem occurs in that the optical system fluctuates its optical characteristic. For example, when it is assumed that for the light having a wavelength of 820 nm, the refractive index of the vacuum atmosphere is 1.00000, and the refractive index of the air atmosphere is 1.00027. This variation of the refractive index influences the optical characteristic of the optical system, such as a focus position, a magnification, and a wavefront aberration.
In general, the optical system to be arranged in the vacuum atmosphere is assembled and adjusted in the air atmosphere. More specifically, a change of the optical characteristic caused by the environmental change, such as an imaging-position changing amount, is previously obtained and adjusted in the air atmosphere. However, the imaging position does not always change by the changing amount as previously calculated due to assembly and adjustment errors or working errors of the optical element, such as scattering of a radius of curvature, a thickness and a glass optical constant of the lens. The optical characteristics of the alignment detection system and focus position detection optical system are likely to remarkably deteriorate in the vacuum atmosphere. In other words, the highly precise detection is precluded when the optical characteristic greatly varies due to the environmental change. Accordingly, an optical system in which its optical characteristic is less likely to change due to the environmental change is proposed. See, for example, Japanese Patent Application, Publication No. 11-271604.
Although this reference discloses an optical system in which the optical characteristic is less likely to change due to the environmental change, the optical system has an infinite object distance. No optical systems or power arrangements having a finite object distance are disclosed. For example, in an attempt to use the optical system disclosed in this reference to configure an optical system that has a finite object distance and the optical characteristic less likely to change the environmental change, two pairs of optical systems having an infinite object distance of this reference should be arranged so that the infinite object side of each optical system can oppose to each other. In addition, these optical systems should be designed to withstand the environmental change. Problematically, the overall length of the optical system would be long, and the number of lenses would increase.
It is conceivable to use a vacuum chamber to adjust the optical system, to measure the optical characteristic change (such as an imaging-position changing amount) in the vacuum atmosphere, and to adjust the air atmosphere based on the measurement result. However, this scheme would be time-consuming and costly, and require complex adjustment faculties.